Thin film stack

ABSTRACT

The present disclosure is drawn to a thin film stack, which includes a substrate, a metal layer, and an adhesive layer comprising a blend of from 3 at % to 94 at % indium oxide, from 3 at % to 94 at % gallium oxide, and from 3 at % to 94 at % zinc oxide. The adhesive layer is adhered between the substrate and the metal layer.

BACKGROUND

Adhesion between various materials in thin film stacks, including the adherence of metals to non-metal films, present challenges, particularly in environments where high temperatures, piezoelectric vibration, and certain migrating elements or compounds may be present in nearby layers. The piezoelectric printhead is an example of such a device that can be prepared or used under some of these conditions. In piezoelectric printheads, for example, various layers of metal and non-metal films are stacked and adhered together; and high temperatures, piezoelectric actuation, and migration of lead or other materials can be common from layer to layer. For example, in systems that use titanium oxide to bond various layers together, lead containing layers that may be present, such as lead zirconate titanate (PZT), provide a source for lead ion migration through metal electrodes into the titanium oxide, which can undermine the function of the device over time. Furthermore, titanium oxide and other adhesive layers tend to underperform when exposed to high manufacturing temperatures. Thus, it would be desirable to provide improved adhesives for use in such thin film stacks and other related thin material systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a portion of inkjet printhead, including a thin film stack, prepared in accordance with examples of the present disclosure;

FIG. 2 is a schematic flow chart depicting a method in accordance with examples of the present disclosure; and

FIG. 3 is a graph depicting surface roughness of a blended adhesive layer of indium oxide, gallium oxide, and zinc oxide as a function of annealing temperature.

DETAILED DESCRIPTION

Piezoelectric devices, such as piezoelectric inkjet printheads or sensors, can be prepared by stacking various piezoelectric materials, other films, and metal, e.g., conductors and/or electrodes, in specific configurations for piezoelectric actuation or piezoelectric sensing. In the case of a piezoelectric printhead, piezoelectric actuation on or in an ink chamber can be used to eject or jet fluids therefrom. One such piezoelectric material is lead zirconate titanate or PZT, which can be grown or otherwise applied on the surface of a metal electrode, such as platinum, ruthenium, palladium, and iridium, as well as some conductive and non-conductive oxides, such as IrO₂, SrRuO₃, ZrO₂, etc. The appropriate adhesion of the metal electrode (which upon completion, can have PZT or another piezoelectric material applied to one side thereof) to an underlying layer can be provided by an adhesion layer including a blend of indium oxide, gallium oxide, and zinc oxide (also referred to herein as “IGZO”). The adhesion of many noble metal, e.g., electrodes, to other materials, such as especially non-metallic surfaces, oxide surfaces, or polymer surfaces, may not typically strong enough without an adhesive layer applied there between. There are also some transition metals, such as copper, that also do not adhere well to some of these same surfaces. Furthermore, even if adhesion is acceptable by using other types of adhesive materials, there can be other drawbacks with some of these other known adhesive materials that are used in thin film stacks, as will be discussed to some degree hereinafter.

In accordance with this, the present disclosure is directed generally towards a thin film stack, which includes a substrate, a metal layer, an adhesive layer adhered between the substrate and the metal layer. The adhesive layer includes a blend of indium oxide, gallium oxide, and zinc oxide, which provides many benefits over other adhesives typically used in the art. A piezoelectric layer can also be attached to the thin film stack, either directly or indirectly through one or more intermediate layer(s). It is noted that the “thin film stack” can also be defined to include the piezoelectric layer. Additionally, other electrodes or layers may also be included as part of the thin film stack. Thus, in describing the thin film stack to include the substrate, the metal layer, and the adhesive layer, these layers are merely set forth to state that the thin film stack includes at least these layers, and other layers can also become part of the thin film stack. Typically, the piezoelectric material can be applied directly to the metal layer, particularly when the metal layer is a metal electrode. Thus, in one example, the metal layer is an electrode, and in another example, the metal layer is a conductive layer. In further detail, particularly in the case of piezoelectric devices, a piezoelectric material can be applied to the metal layer.

In another example, a method of preparing a piezoelectric thin film stack can comprise adhering a first side of a metal layer to a substrate using an adhesive blend of indium oxide, gallium oxide, and zinc oxide, and forming a piezoelectric material on a second side of the metal layer.

In each of the various embodiments described herein, whether discussing the thin film stack or related methods, there may be some common features of each of these embodiments that further characterize options in accordance with principles discussed herein. Thus, any discussion of the thin film stack or method, either alone or in combination, is also applicable to the other embodiments not specifically mentioned. For example, a discussion of the adhesive layer in the context of the thin film stack is also applicable to the related method, and vice versa.

It is noted that when referring to the various oxides, such as “indium oxide” or “gallium oxide,” these terms can include any of various blends of the oxidized metals. For example, indium oxide is typically found in the form of indium sesquioxide (In₂O₃), and gallium oxide is typically in the form of gallium sesquioxide (Ga₂O₃). However, other forms can be present. Furthermore, the blend of indium oxide, gallium oxide, and zinc oxide can also be referred to herein as In₂Ga₂ZnO₇, or more generically as IGZO. This is because these materials, when blended, do not typically all retain their discrete molecular form, i.e. the adhesive layer blend can become more integrated chemically into a more complex structure. Additionally, there can also be multiple forms of oxides under certain conditions. Thus, determining the atomic % (at %) of the IGZO blend in the adhesive layer, the percentage can be determined based on the total atomic percentage of each respective element, i.e. indium, gallium, zinc, and oxygen. For example, if a composition includes 100 at % IGZO (with essentially no dopant), then there could be about 16.67 at % indium, about 16.67 at % gallium, about 8.33 at % zinc, and about 58.33 at % oxygen. If there is dopant present (anything other than indium, gallium, zinc, and oxygen), then the percentages of each of these elements would decrease accordingly. On the other hand, when determining the relative atomic percentage of the indium oxide, gallium oxide, and zinc oxide relative to one another, the metal oxide can form the basis of the atomic percentage (or molecular percentage of the oxide compound). Thus, in stating that indium oxide is present in the adhesive layer from 3 at % to 94 at %, this means that the indium oxide molecule is present in the adhesive layer at from 3 at % to 94 at % based on the composition as a whole. As a result, in accordance with the present disclosure, atomic percentages can be calculated not only for individual elements, but also for metal oxides, taking into account both the metal content and the oxygen content. This can also be referred to as “molecular percent” but for consistency, atomic percent is used herein throughout and the context will determine whether the percentage is based on the individual element or on the small molecule.

It is further noted that when referring to “titanium oxide” for comparison purposes, it is understood that there are various forms of titanium oxides, including TiO₂, Ti₂O₃, TiO, etc., and thus, the general term “titanium oxide” is used to include adhesives or layers that generally include oxidized titanium.

Turning now to the FIGS., it is noted that the drawings herein are not to be considered as being to scale, and are thus, merely schematic to assist in showing and describing an embodiment of the present disclosure. Furthermore, though an inkjet printhead is shown in the FIGS. with various specific layers, it is understood that this is not intended to limit the scope of the present disclosure. This example is merely provided to show an example of the various thin film stacks that can be used in various devices, such as piezoelectric actuators or sensors.

FIG. 1 sets forth a schematic view of a portion of an inkjet printhead 100, with an expanded view of Section A provided for additional clarity. In this FIG, a silicon support is fabricated to include multiple ink chambers 112 for receiving and jetting ink there from. It is noted that often, ink chambers or other areas where ink may contact the printhead can be coated with any of a number of protective coatings. Those coatings are not shown in FIG. 1, but it is understood that such a coating may be used for protective purposes without departing from the scope of the present disclosure. For example, tantalum or tantalum oxide coatings, e.g., Ta₂O₅, are often used for this purpose. Other support material(s) can be used alternatively or in addition to the exemplified silicon support and optional protective coatings. Thus, the term “support” typically includes structures comprising semi-conductive materials such as silicon wafer, either alone or in assemblies comprising other materials applied thereto. Metallic supports can also be used, including metallic materials with an insulating material applied thereto. Certain specific materials that can be used for the support material include silicon, glass, gallium arsenide, silicon on sapphire (SOS), epitaxial formations, germanium, germanium silicon, diamond, silicon on insulator (SOI) material, selective implantation of oxygen (SIMOX) substrates, or other similar materials. Furthermore, the substrate described herein can actually be the support material, particularly when the support material inherently includes an oxidized surface. However, in many typical examples, a separate membrane of oxidized material is applied to the support and acts as the substrate for enhanced adhesion. Another reason that such a membrane might be included is to prevent diffusion of the metal into a silicon substrate, for example.

Turning now to the thin film stack 100 of this example, a substrate 114, an adhesive layer 116, a metal electrode 118, a piezoelectric layer 120, a second metal electrode 122, and a passivation layer 124 are shown. Other layers are also usable, including other insulating, semi-conducing, conducting, or protective layers that are not shown. However, one skilled in the art would recognize other layers that could optionally be used, or optionally omitted from the structure shown in FIG. 1.

In the system shown, the metal electrode 118 and the second metal electrode 122 are used to generate an electric field with respect to the piezoelectric layer 120, and as the piezoelectric layer is actuated, the thin film stack 100 vibrates into an appropriate ink chamber 112, causing inkjetting to occur. As each of these layers are shown in this example, each will be described hereinafter in additional detail, with emphasis placed on the adhesive layer and its interaction with the substrate, e.g., oxide layer, and the metal layer that the adhesive layer contacts.

Turning to the specific layers, with specific reference to the substrate 114, this layer can be the support material with an oxide layer inherently present on its surface, but is typically prepared as an oxide membrane applied to the support material, e.g., SiO₂, ZrO₂, HfO₂, Ta₂O₅, Al₂O₃, SrTiO₃, etc. These membranes can be applied as multiple layers, and/or be prepared using multiple materials in a common layer. Thus, the materials are typically applied as one or more layer to the silicon or other support material as described above. When the substrate is in the form of a thin film or membrane, the substrate can be formed at a thickness from 10 Å to 10 μm, for example. In an example piezoelectric actuator device, the thickness of this substrate, e.g., oxidized membrane, can be approximately the same thickness as piezoelectric layer, e.g., at a 1:2 to 2:1 thickness ratio of substrate layer to piezoelectric layer, and both layers can be about 50 nm or greater.

In further detail with respect to the metal layers, which are metal electrodes 118, 122 in this example, these electrodes can be applied at a thickness from 5 nm to 5 microns, though thicknesses outside this range can also be used. Materials that can be used, particularly for electrodes, typically include noble metals or other metals or alloys, including but not limited to, platinum, copper, gold, ruthenium, iridium, silver, nickel, molybdenum, rhodium, and palladium. In other examples, oxides of these or other metals can also be used, such as IrO₂ or SrRuO₃, if the adhesive properties of the adhesion layers of the present disclosure would be beneficial for use. Platinum is of particular interest as a metal that benefits from the adhesive layers of the present disclosure because its surface does not readily become oxidized. Metal electrodes (or metals applied for another purpose, such as for conductive layers or traces) can be deposited using any technique known in the art, such as sputtering, evaporation, growing the metal on a substrate, plasma deposition, electroplating, etc.

In piezoelectric systems in particular, the metal electrodes 118, 122 selected for use should be those which can effectively cause appropriate movement of the piezoelectric materials, such as those used in the piezoelectric layer 120. This can be particularly true with respect to metal electrode 118, which is often in direct contact with the adhesive layer 116. To illustrate, in one example, the piezoelectric layer can comprise lead zirconate titanate (PZT). As PZT contains lead, and lead cations are migratory though other metals under the proper conditions, there can be problems associated with lead migrating into and through the metal electrode, e.g., lead migrates fairly readily through platinum when a titanium oxide adhesive layer is applied to the opposite side of the metal electrode. This is believed to occur because after annealing platinum and the titanium oxide during the manufacturing process, especially at high temperatures, lead cations can diffuse into and through the platinum and into the titanium oxide, forming lead titanate (PbTiO₃). Thus, in accordance with examples of the present disclosure, by using a blend of indium oxide, gallium oxide, and zinc oxide decreased migration of lead cations through the metal electrode compared to similar systems utilizing titanium oxide adhesive can be achieved.

As alluded to, with respect to inkjet printheads or fluid ejection systems, the piezoelectric film or layer is typically disposed between the electrodes 118, 122 and receives drop firing and non-firing signals from a controller acting on the electrodes. Thus, the electrodes are actively driven to actuate the piezoelectric layer, thereby causing ink to flow from the ink chamber 112 to a drop forming outlet or orifice (not shown). The inkjet ink is thus jetted toward a print medium or other transfer surface or device. During this firing process, piezoelectric actuators often can be fired up to 100 billion cycles, or even higher, which even at relatively low temperatures, e.g., less than 100° C., can lead to diffusion of lead cations over time into and through the metal electrodes. Furthermore, and perhaps more significantly, during manufacture of certain piezoelectric devices, very high temperatures are often used, e.g., exceeding 500° C., 700° C., or even 1000° C. Thus, lead cation diffusion from a piezoelectric layer, such as PZT, into/through a metal electrode, such as platinum, can be relatively quick and problematic. As mentioned, this is particularly problematic when titanium oxide is used as the adhesive on the opposite side of the metal electrode as the PZT, and use of the adhesive blends of the present disclosure provide a solution to this problem.

A suitable material for the piezoelectric layer 120 that can be used includes, as mentioned, lead zirconium titanate (PZT). In general, with respect to PZT, the general formula can be Pb(Zr_(1-x)Ti_(x))O₃, where x is from 0.1 to 0.9. However, it is notable that different dopants can be used, such as La, Nb, etc. Thus, other materials for the piezoelectric layer can also be used, including lead lanthanum zirconium titanate (PLZT, or La doped PZT), lead niobium zirconium titanate (PNZT, or Nb doped PZT), and PMN-PT (Pb(Mg,Nb)O₃—PbTiO₃). Lead-free piezoelectric layers may also be used, examples of which include LiNbO₃, BCTZ [Ba(Ti_(0.8)Zr_(0.2))O₃—(Ba_(0.7)Ca_(0.3))TiO₃], tungsten bronze structured ferroelectrics (TBSF), BNT-BT [(Bi_(0.5)Na_(0.5))TiO₃—BaTiO₃], BT [BaTiO₃], AlN, AlN doped with Sc, and ternary compositions in the BKT-BNT-BZT [(Bi_(0.5)K_(0.5))TiO₃—(Bi_(0.5)Na_(0.5))TiO₃—Bi(Zn_(0.5)Ti_(0.5))O₃] system, a specific example of which includes 0.4(Bi_(0.5)K_(0.5))TiO₃-0.5(Bi_(0.5)Na_(0.5))TiO₃-0.1Bi(Zn_(0.5)Ti_(0.5))O₃). Other suitable piezoelectric materials can be used for the piezoelectric layer, or combinations of materials or multiple layers can likewise be used in accordance with examples of the present disclosure.

Turning now to more specific detail regarding the adhesive layer 116, as mentioned, this layer can comprise a blend of indium oxide, gallium oxide, and zinc oxide, or IGZO. The blend of indium oxide, gallium oxide, and zinc oxide can be applied at various atomic ratios (based on the atomic ratio of the cation). To illustrate, in one specific, an atomic blend of indium:gallium:zinc can be 2:2:1. Typically, this blend, based on the oxides, e.g., indium oxide:gallium oxide:zinc oxide, will be about 1:1:1 because IGZO typically has a general blend formula of about In₂Ga₂ZnO₇. More generally, the indium oxide, the gallium oxide, and the zinc oxide can each be present in the adhesive from 3 at % to 94 at % based on the metal oxide content. In another example, the indium oxide, the gallium oxide, and the zinc oxide can each be present in the adhesive from 10 at % to 80 at %, based on the metal oxide content. In still another example, the indium oxide, the gallium oxide, and the zinc oxide can each be present in the adhesive from 20 at % to 60 at %, based on the metal oxide content.

When applied, the adhesive layer is typically at a thickness from 10 Å to 0.5 μm, and often, applied at a thickness ranging from 50 Å to 1000 Å. In one example, the blend of indium oxide, gallium oxide, and zinc oxide can substantially make up the entirety of the adhesive layer (essentially 100 at % of the layer based on a calculation of indium, gallium, zinc, and oxygen), but practically, a range from 90 at % to 100 at % can be typical (with up to 10 at % of dopant based on a calculation of any element other than indium, gallium, zinc, and oxygen). Thus, dopant(s) can be included in the layer, and comprise elements or compounds containing elements, such as Pb, Sb, Fe, Cu, Ca, In, Ga, Cd, Ge, Ti, etc. Ranges of dopant can be from 0.05 at % to less than 3 at % for individual elements, or from 0.05 at % to less than 3 at % for dopant compounds. Multiple dopants can be present, and thus, can exceed 3 at % cumulatively in some cases, but any single dopant, if present, is included at less than 3 at % or it is not considered to be a dopant. With respect to the adhesive layer, the indium oxide, gallium oxide, or zinc oxide, by definition, are not considered to be dopants. Thus, in the adhesive layer, any cation or other atom included that is other than indium, gallium, zinc, or oxygen should be considered as a dopant.

In further detail regarding the adhesive layer 116, it is noted that the blend of indium oxide, gallium oxide, and zinc oxide can be amorphous. As an amorphous compound, the adhesion blend promotes uniform mechanical performance, typically produces an atomically smooth interface, and provides acceptable barrier properties to lead and other impurities that may migrate into the adjacent electrode. However, with respect to this particular blend of indium oxide, gallium oxide, and zinc oxide, even at higher temperatures where some crystallization begins to occur, e.g., about 28% by volume at 1000° C. in one example, the crystallization tends to occur in the center of the layer, leaving the outer surfaces of the adhesive blend to remain in a more amorphous state. Thus, the adhesive layer 116 is shown schematically as having a first amorphous layer 116 a attached the metal layer 118, a second amorphous layer 116 c attached to (and sometimes admixed to some degree with) the substrate 114, and a crystalline center region 116 b. Thus, even at 1000° C., when crystallization occurs fairly significantly, the crystallization remains primarily in the central region, which causes the amorphous surfaces of the adhesive layer to remain smooth and adhesive to the respective layers to which they are bound.

Thus, the adhesive layer of the present disclosure provides reliable adhesion between many noble and other metallic materials, including platinum, palladium, copper, gold, ruthenium, iridium, etc. Furthermore, acceptable adhesion to non-metallic materials can also be achieved, making it a good adhesive to use between metallic and non-metallic layers or surfaces. As also mentioned, blends of indium oxide, gallium oxide, and zinc oxide provide an acceptable diffusion barrier compared to titanium oxide. More specifically, even though titanium oxide works as an acceptable barrier in preventing lead from passing through the adhesive titanium oxide itself, it does not act sufficiently well to prevent migration of the lead into and through some metal electrodes, such as platinum. As lead reacts with titanium oxide to some degree to form lead titanate, even though the lead is stopped in the titanium oxide layer, it is still drawn through the metal electrode in order to contact the titanium oxide layer and react therewith. Specifically, with titanium oxide adhesion layers, a common lead content can be around 3 at % (or 1:10 ratio of Pb to Ti). Lead titanate (PbTiO₃) has a 1:1 atomic ratio, and thus, there is a lot of room for the attraction of lead by titanium oxide if the conditions are right, e.g., depending on the thickness, grain size/grain density of metal electrode layer, choice of metal, temperature and duration of anneal, etc. On the other hand, blends of indium oxide, gallium oxide, and zinc oxide do not have this same reactivity with lead, and thus, there is little to no migration into the metal electrode in the first place, as there are no reactive conditions awaiting the lead on the other side of the metal electrode. Thus, lead tends to stay out of the bulk of platinum layer altogether.

The blend of indium oxide, gallium oxide, and zinc oxide of the present disclosure also provides a reliable adhesion mechanism for adhering noble metals to non-metallic membranes, such as oxide substrates, while remaining thermally stable at higher temperature than many other adhesion layer materials. This, in combination with providing an improved lead diffusion barrier into the metal electrode, particularly when heated to high temperatures, provides a significant advantage over other adhesion systems. These and other properties promote a good combination of benefits provided by the blend of indium oxide, gallium oxide, and zinc oxide described herein. For example, with respect to platinum and other similar electrodes, there are thermophysical instabilities at temperatures in excess of 700° C. as a result of low wettability, leading to delamination. These defects often cause degraded performance and can result in inoperable devices.

Another advantage of the adhesive layer of the present disclosure is related to surface roughness, i.e. maintaining a relatively high level of smoothness, even at very high manufacturing temperatures. Generally, as processing temperatures are increased, the surface roughness can increase as well with many adhesives. However, with the adhesive layers of the present disclosure comprising blends of indium oxide, gallium oxide, and zinc oxide, the surface roughness, even with very high annealing temperatures, e.g. 1000° C. or more, can be low enough to still be effective for processing and use. This is partly due to the effect described previously where crystallization tends to stay at the center of the layer, and the outer surfaces (that typically contact the substrate and the metal layer) tends to remain amorphous. To provide an example of the benefits of this and other characteristics related to smoothness, when annealed at 1000 C.° for 0.5 hours, a 1.1 nm RMS roughness was observed and an even lower roughness was observed when the material was annealed for a shorter time using Rapid Thermal Processing (RTP). This demonstrates the roughness suppression that is provided by this adhesive layer material. More specifically for comparison purposes, this surface roughness less than that observed for many sputter deposited metal films of similar thickness, which was unexpected. Thus, when attaching a substrate to a metal electrode, such as a silicon dioxide layer to a platinum electrode, surface smoothness that can be maintained over a wide variety of processing conditions, which is particularly useful in preparing PZT with respect to texture and piezoelectric response.

As a final note with respect to FIG. 1, a passivation layer 124 is shown, and can be formed of any suitable material, including, but not limited to wet or dry process silicon dioxide, aluminum oxide (e.g., Al₂O₃), silicon carbide, silicon nitride, tetraethylorthosilicate-based oxides, borophosphosilicate glass, phosphosilicate glass, or borosilicate glass, HfO₂, ZrO₂, or the like. Suitable thicknesses for this layer can be from 10 nm to 1 μm, though thicknesses outside of this range can also be used.

With respect to the various layers described herein, any of a number of deposition methods or techniques can be used. For example, as mentioned, a PZT layer can be grown on or otherwise applied to the surface of a metal in some examples. Deposition techniques that can be used for depositing piezoelectric material or other layers on top of one another include sol-gel deposition, physical vapor deposition (PVD), pulsed laser deposition (PLD), atomic layer deposition (ALD), etc. Metal can be deposited, for example, by sputtering or other known deposition methods. Semi-conductive, non-conductive, or passivation layers can be deposited by plasma enhanced chemical vapor deposition (PECVD), a low pressure chemical vapor deposition (LPCVD), an atmosphere pressure chemical vapor deposition (APCVD), atomic layer deposition (ALD), sputter deposition, evaporation, thermal oxide deposition, or other known methods. Any combination of these or other methods can be used.

Turning to FIG. 2, a method 200 of preparing a piezoelectric device can comprise adhering 210 a first side of a metal layer to a substrate using an adhesive blend of indium oxide, gallium oxide, and zinc oxide, and forming 220 a piezoelectric material on a second side of the metal layer. In one example, the piezoelectric material is PZT and is grown or otherwise applied on the first side of the metal layer. Normally, the adhesion layer goes on the substrate, and then the metal is applied to the adhesion layer (which is typically annealed prior to the metal deposition). The piezoelectric material, such as the PZT, can then be deposited layer by layer (if sol-gel is used) or the entire PZT film can be deposited in case of physical vapor deposition (PVD), pulsed laser deposition (PLD), or combinations thereof, for example. Other piezoelectric materials can likewise or alternatively be applied, as has been described previously.

EXAMPLES

The following examples illustrate the adhering, barrier, and structural properties of the present disclosure. However, it is to be understood that the following are only exemplary or illustrative of the application of the principles of the present devices and methods. Numerous modifications and alternative devices and methods may be devised by those skilled in the art without departing from the spirit and scope of the present compositions and methods. The appended claims are intended to cover such modifications and arrangements. Thus, while the present examples have been described above with particularity, the following provides further detail in connection with what are presently deemed to be the acceptable embodiments.

Example 1

A 1:1:1 molecular ratio blend of indium oxide, gallium oxide, and zinc oxide (IGZO, In₂Ga₂ZnO₇), by metal oxide molecular ratio adhesive, was prepared and its interaction with thin film SiO₂ was investigated. The purpose of this study was to demonstrate the inherent thermal stability of the blends of indium oxide, gallium oxide, and zinc oxide in thin film stacks of the present disclosure. The structure was subjected to annealing temperatures up to 1000° C. in a furnace and in an RTP (rapid thermal annealing) tool with rates of heating of 10° C./min and 50° C./sec, respectively. The duration was 0.5 hour in a furnace and 10 min in RTP. The surface roughness was evaluated using Atomic Force Microscopy (AFM), and characterized as Root Mean Square (RMS) Roughness, nm. As can be seen in FIG. 3, the surface roughness of the IGZO after annealing in a furnace at 1000° C. is about 1.1 nm. IGZO roughness after RTP at 1000° C. was even lower, around 0.5 nm.

Example 2

For comparison purposes in determining lead diffusion or migration through a thin metal electrode, two similar structures were prepared, the only difference being the choice of adhesive layer. Specifically, both structures included platinum electrode adhered to a silicon dioxide layer with the adhesive layer applied there between. To the platinum electrode was also deposited a layer of PZT (opposite the silicon dioxide layer). This material stack was exposed to thermal processing in excess of 700° C. during the PZT processing steps. The final thickness of the PZT (PbZr_(0.52)Ti_(0.48)O₃) was around 1 micron. In the first structure, the adhesive layer was a 50 nm thick blend of indium oxide, gallium oxide, and zinc oxide (IGZO, 1:1:1 atomic ratio based on the ratio of the metal oxides). In the second structure, around 20 nm of a titanium oxide was used as the adhesive.

Analysis was then completed to determine how much lead each structure permitted to pass through the platinum electrode into the adhesive layer, as observed by XPS analysis. For this analysis, the samples were used after selective PZT etching, analyzing only platinum electrodes applied to substrates with the adhesion layer applied there between. In case of the titanium oxide adhesion layer, lead was detected in quantities of at least 3 at %, which corresponds to 10:1 ratio between Ti and Pb or Pb_(0.1)TiO_(2.1). Lead was not detectable with the XPS instrumentation below the PZT in either the platinum or the IGZO adhesion layer.

Example 3

The level of crystallinity (as a percentage by volume) of IGZO was determined at various annealing temperatures (furnace at 10° C./min, 30 min duration). Specifically, grazing incidence angle x-ray diffraction (GIXRD) analysis was performed using a Philips MRD system employing Cu Kα radiation (1.54056 Å). Diffraction data for the sample was corrected for background fluorescence and noise using a Sommerfield-Visser algorithm with a weighting factor of 0.45 and diffraction intensity from scattering of Kα₂ radiation, which was subtracted from the data set using the Rachinger method with a Kα₁/Kα₂ ratio of 0.43. In this experiment, the CHI laboratory reference angle was oscillated between −2.0 and +2.0 degrees and the PHI laboratory reference angle was oscillated by 360 degrees to assure that all crystallographic texture present in the material was captured. The analytical beam size in this experiment was about 1 cm².

The data collected is provided in Table 1, as follows:

TABLE 1 XRD Data ANNEAL % CRYSTALLINITY TEMPERATURE (BY VOLUME) As Deposited ~1 500° C. <2 600° C. <2 700° C. 7 800° C. 12 900° C. 19 1000° C.  28

With respect to the XRD data shown in Table 1, the crystalline phases detected in the “As Deposited” material are In₂O₃ of cubic symmetry and Ga₂O₃ of monoclinic symmetry. As the annealing temperature was increased, the percent crystallinity of the material increased, accompanied by a transition from discrete reagent phases to a fairly homogenous mixture of two discrete solid solution phases, namely a phase with rhombohedral symmetry and a phase with hexagonal symmetry. The hexagonal phase of the solid solution was the first to begin active crystallization at from 600° C. to 700° C. The rhombohedral phase appears to begin nucleation somewhere between 800° C. and 900° C. Even at 1000° C., there is only about 28 vol % crystallinity, and as shown in FIG. 1 at 116 a-c, the crystallinity tends to be centrally located in the adhesive layer, with at least a top portion of the layer remaining amorphous. Thus, the amorphous surfaces provide an exceptionally smooth surface for adhesion between both the substrate and the metal layer, as described herein.

While the disclosure has been described with reference to certain embodiments, those skilled in the art will appreciate that various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the disclosure. It is intended, therefore, that the present disclosure be limited only by the scope of the following claims. 

What is claimed is:
 1. A thin film stack, comprising a substrate, a metal layer, and an adhesive layer adhered between the substrate and the metal layer, and a piezoelectric layer, the adhesive layer comprising from 3 at % to 94 at % indium oxide, from 3 at % to 94 at % gallium oxide, and from 3 at % to 94 at % zinc oxide.
 2. The thin film stack of claim 1, wherein the piezoelectric layer is attached directly to an opposing surface of the metal layer with respect to the adhesive layer, the metal layer is a metal electrode, and the thin film stack further comprises a second metal electrode positioned on the piezoelectric layer such that when the metal electrode and the second metal electrode are used to generate an electrical potential, the piezoelectric layer acts as a piezoelectric actuator.
 3. The thin film stack of claim 1, wherein the piezoelectric layer is PZT.
 4. The thin film stack of claim 1, wherein the thin film stack is an actuator for a fluid ejection device.
 5. The thin film stack of claim 1, wherein the substrate comprise a membrane from 10 Å to 10 μm in thickness, and the membrane is selected from the group of nitrides, carbides, oxides, and metals.
 6. The thin film stack of claim 1, wherein the metal layer is a metal electrode from 5 nm to 5 microns in thickness, and is selected from the group of platinum, silver, iridium, palladium, and alloys thereof.
 7. The thin film stack of claim 1, wherein the blend of indium oxide, gallium oxide, and zinc oxide comprises from 90 at % to 100 at % of the adhesive layer.
 8. The thin film stack of claim 1, wherein the adhesive layer is from 10 Å to 0.5 μm in thickness.
 9. The thin film stack of claim 1, wherein the adhesive layer, in addition to the blend, comprises from 0.05 at % to less than 3 at % of at least one dopant.
 10. The thin film stack of claim 1, wherein the adhesive layer provides adhesion between the substrate and the metal layer up to at least 1000° C. without delamination.
 11. The thin film stack of claim 1, wherein the adhesive layer comprises a first amorphous layer attached to the metal layer, a second amorphous layer attached to the substrate, and a crystalline layer therebetween.
 12. A method of preparing a piezoelectric thin film stack, comprising: adhering a first side of a metal layer to a substrate using an adhesive blend of from 3 at % to 94 at % indium oxide, from 3 at % to 94 at % gallium oxide, and from 3 at % to 94 at % zinc oxide; and forming a piezoelectric material on a second side of the metal layer.
 13. The method of claim 12, wherein the piezoelectric material is PZT and is applied to the second side of the metal layer.
 14. The method of claim 12, wherein the blend of indium oxide, gallium oxide, and zinc oxide comprises from 90 at % to 100 at % of the adhesive blend. 